Shell low (never as high as wide), the apex not prominent and often
not even distinct; exterior whitish or with a light periostracum, but not
dark brown; frequently on empty shells of bivalves or gastropods, including
those occupied by hermit crabs, but not attached to the shells of living
Tegula,
Calliostoma, or other gastropods

Shell whitish, generally almost flat, with a shaggy, yellow-brown periostracum

Crepidula nummariaGould, 1846

5b

Shell whitish, almost flat, concave, or misshapen (depending on habitat),
with a thin, brown periostracum or without any obvious periostracum (often
in burrows of pholadid bivalves, in empty shells, and inside shells occupied
by hermit crabs, fitting the contour of the concave surface)